Salt bridges

 In proteins, salt bridges occur between amino acid side-chains with opposite positive or negative full-electron charges, namely, (at neutral pH) Glu- or Asp- vs. Arg+ or Lys+. They may also occur between ionized organic ligands, such as acetylcholine+ (or example at right: 1cbr), or inorganic ions, such as K+ or Cl-, and amino acid side-chains.

A salt bridge is generally considered to exist when the centers of charge are 4 &Aring; or less apart. The center of charge of the arginine sidechain is the zeta carbon. The energetic significance of such complementary charge pairs is a complex function of the local environment.

Putative salt bridges can be displayed by FirstGlance in Jmol.